Electric lighting and power controllers therefor

ABSTRACT

In a lighting power controller having a controllable switch (such as a thyristor), in order to compensate for perturbations in the mains supply waveform, the waveform is analysed and a table is set up of thyristor firing angle against output RMS voltage. To obtain a desired output RMS voltage, the thyristor is fired at the angle indicated by the table.

This application is a division of application Ser. No. 07/450,294 filedDec. 12, 1989 now U.S. Pat. No. 5,066,896.

This invention relates to electrical power controllers which are for usein an AC circuit to control a lighting load and which particularly, butnot exclusively, employ a controllable switch which is operated so as toconduct during part of half cycles of the AC supply. The invention ismore particularly, but not exclusively, concerned with lighting circuitsincluding luminaries for stage, or television or film studio, lighting.

A tungsten filament electric lamp functions essentially as a black bodyradiator, and accordingly the spectral characteristics of the lamp aredependent upon the temperature of the filament and thus upon the appliedRMS voltage. Especially in a colour television studio, great attentionis paid by the camera operator to the colour balance of the camera toachieve faithful reproduction, especially of skin tones. This colourbalance is altered by variations in the lighting colour temperature.Such variations can be caused, amongst other things, by variation in thevoltage of the mains supply. In a perfect system, the mains voltagewaveform is a perfect sine wave, having a peak amplitude equal to thesquare-root-of-two times the rated RMS voltage. However, in practice,the amplitude may be less due to voltage drops in the supply cable, andthe sine wave form may be distorted in two main ways. Firstly, the peaksof the wave may be suppressed due to saturation of transformers used inthe supply network. Secondly, at a site where many thyristor or triacdimmer controlled loads are in use, the form of each half wave may bereduced in a final portion of the half wave due to the increased load onthe supply compared with the initial portion of the half wave.

The present invention is concerned with compensating for perturbationsin the mains supply, and in accordance with one aspect of the inventionthe mains voltage is measured during a half-wave cycle, the firing pointfor a thyristor or triac which would provide a desired output RMSvoltage is determined, and the thyristor or triac is fired accordingly.

Preferably, the invention is performed by forming a table of measuredvoltage against time, processing the data of the table to form anothertable of RMS output voltage against firing point, and then inverting thelatter table to provide a look-up table of firing point against RMSoutput voltage.

There follows a description by way of example of a specific embodimentof the invention and modifications thereto, with reference to theaccompanying drawings in which:

FIG. 1 is a block diagram of a lighting system;

FIG. 2 is an equivalent power circuit for each dimmer channel;

FIG. 3 is a block diagram of one of the dimmer processors of FIG. 1;

FIGS. 4A and 4B are flow charts of the processes carried out by thedimmer processor of FIG. 3;

FIG. 5 is a block diagram of one of the dimmer units of FIG. 1;

FIG. 6 is a block diagram of a modified dimmer processor;

FIG. 7 is a flow chart of the processes carried out by the dimmerprocessor of FIG. 6;

FIG. 8 is a voltage-time graph of a mains half-cycle;

FIG. 9 is a block diagram of the mains compensation processor of FIG. 1;and

FIG. 10 is a flow chart of the process carried out by the mainscompensation processor of FIG. 9.

Referring to FIG. 1, a lighting control system is shown which includes alighting control desk 10 having a common processor unit 12, a data inputterminal 14 and a bank of faders 16 for respective dimmers. The commonprocessor unit 12 sends data to one or more dimmer processors 18, two ofwhich are shown for simplicity. Each dimmer processor controls one ormore dimmers 20, two of which are shown for each dimmer processor 18.Each dimmer 20 is connected in series with a load 22 across a mainssupply L-N and is associated with a respective current sensor 24.

Referring to FIG. 2, an equivalent power circuit is shown for eachdimmer channel. An RMS voltage Vi is supplied by the mains L-N to acontrollable switch, such as a thyristor 26, which is closed part-waythrough each mains half-cycle and opens at the end of the cycle,producing a switched output RMS voltage Vs. A current-independent RMSvoltage drop Vd arises across the thyristor 26. The thyristor 26 andassociated dimmer components such as a filtering inductor also act asresistor, represented by Rd, across which there is an RMS voltage dropIoRd, where Io is the RMS output current. The connecting cable of thecircuit also acts as a resistor, represented by Rc, across which thereis an RMS voltage drop IoRc. It will therefore be appreciated that theRMS voltage Vl across the load 22 will be:

    V=Vs-Vd-Io (Rd+Rc)

and that Vs will be a function of the supply voltage and the conductionperiod in each half cycle of the switch 26.

Referring to FIG. 3, there is shown a block diagram of one of the dimmerprocessors 18. The processor includes an input/output port 28, whichreceives digital signals C1, C2, representing the settings of thedesired levels for the respective dimmer channels 1 and 2. The signals Cfor all of the dimmer channels may be transmitted from the processor ofthe control desk as time-division-multiplexed signals, or as signalsassociated with addresses of the respective channels, all on a singleline. Alternatively, the control signals C may be transmitted as digitalor analogue signals on separate lines. The input port also receivesoutput current signals Io1, Io2 from the respective dimmers 20, andsupplies timing signals T1, T2 to the respective dimmers.

The dimmer processor 18 also includes a microprocessor 30, a program ROM32, and a RAM which stores various tables and variable values. For eachdimmer channel there is a look-up table 34 which relates RMS loadvoltage Vl to control value C (only one table 34 is shown forsimplicity). In common for all dimmer channels controlled by therespective dimmer processor, there are (a) a look-up table 36 whichrelates predicted RMS current Ip' to the RMS load voltage Vl for atungsten filament load of predetermined rating, for example 1kW; and (b)a look-up table 38 which relates thyristor conduction angle A to theswitched output RMS voltage Vs. In common for all of the dimmerchannels, the RAM stores a value f of the mains frequency, and for eachdimmer channel it stores the resistance values Rd, Rc and thyristorstatic voltage drop value Vd, mentioned above, and also a value W of thepower of the respective load 22.

For each dimmer channel, the dimmer processor 18 performs two processesas shown in FIGS. 4A and 4B. FIG. 4A shows a feed-forward loop forreceiving the control signal C and outputting the timing value T. Instep 40, the value of C is taken from the I/O port 28. In step 42, thetable 34 is used to look-up the RMS load voltage Vl to be supplied forthe value C. In step 44, the table 36 is used to look-up an RMS currentIp' which it is predicted would flow if the load were a 1kW tungstenfilament lamp. In step 46, the value Ip' is scaled by the factor W whichis the curently stored value of the power of the load (in kW) to obtainthe predicted current Ip to the load. In step 48, the required switchedoutput RMS voltage Vs is calculated using the equation mentioned abovewith reference to FIG. 2 and the stored values of Rd, Rc and Vd. In step50, the table 38 is used to look-up the firing angle A which is requiredto provide the calculated switched voltage Vs. In step 52, the firingtiming T after the start of a half-wave cycle is calculated from theequation T=A/(2πf) using the stored value of f. In step 54, thecalculated value T is sent via the I/O port 28 to the respective dimmer20. The process is then repeated.

FIG. 4B shows a feed-back process performed by the dimmer processor 18.In step 56, the value Io of the measured output current is taken fromthe I/O port 28. It is then determined in step 58 whether the measuredcurrent Io is equal to the predicted current Ip utilised in the processof FIG. 4A. If so, the process of FIG. 4B loops back to the beginning.However, if there is an inequality, in step 60 the stored load value Wis incremented by an amount proportional to the difference betweenmeasured load current Io and the predicted load current Ip. The processthen loops back to the beginning.

Reference is now made to FIG. 5 which illustrates one of the dimmers 20.A pair of thyristors 62', 62" are connected oppositely in parallel inthe power line from the mains supply to the load. An inductor 64 isincluded for filtering, and a current sensor 66, for example in the formof a multi-turn coil of wire, is placed on the load side of thethyristors and provides a analogue signal proportional to the loadcurrent. The dimmer also includes a circuit 68 including ananalogue-to-digital converter 70 to convert the detected current signalto a digital value Io and a register 72 for storing the detected currentvalue. An input/output port 74 is included for outputting the detectedcurrent value Io to the dimmer processor 18, and for receiving from thedimmer processor 18 the firing time value T in the form of 10 bit data,which is passed to a timing register 76. The circuit 68 also includes aten bit timebase 78 controlled by a crystal 80. The timebase 78 is resetby a zero-crossing signal provided by a zero-crossing detector 82connected to the supply line. Resetting occurs at the beginning of eachhalf-cycle of the mains. The outputs of the timebase 78 and the timingregister 76 are compared by a comparator 84, and once the timebaseoutput has increased so as to equal the content of the timing register76, a signal is provided to a driver circuit 86 which suppliesappropriate pulses to the gates of the thyristors 62', 62" so that theappropriate thyristor conducts for the remainder of the half-cycle. Thusthe switch controlling means, in the form of the driver circuit 86, isoperated to increase the conduction period of the switch constituted bythe thyristors 62', 62", as the current represented by the currentsignal Io increases.

It will be appreciated from the above that for each dimmer channel therespective dimmer processor provides a conversion from the control valueC to the firing timing T taking into account the desired dimmer transfercharacteristic (Table 34) and the voltage drop in the circuit. Thevoltage drop is calculated on the basis of a predicted current in orderto avoid high errors in compensation due to transmission delays and toprocessing delays in the event of the control value C being rapidlychanged. For example, if the control value C is suddenly increased froma minimum value to a maximum value, a current higher than the steadystate current will initially flow through the lighting load, until thesteady state temperature and resistance of the lamp filament arereached. If the voltage drops were determined from the measured current,rather than the "predicted" current, then until the high transientcurrent value has been measured, transmitted and processed,under-compensation would be provided for the voltage drop in thecircuit. Once the high transient current had been measured andprocessed, over-compensation would be provided, because by that time thetransient would have passed and the steady state reached. By utilising a"predicted" current determined from the filament characteristic (Table36) and the stored load, the errors in compensation during transientsare reduced, and by adjusting the stored load value (FIG. 4B), steadystate compensation is correctly achieved.

It is possible that, in some applications, the errors in compensationdescribed above could be minimised and tolerated. In this case, asimplified system can be used, in which the dimmer processor is modifiedas shown in FIG. 6 and performs a single process as shown in FIG. 7,rather than the two processes shown in FIGS. 4A and 4B. The dimmerprocessor of FIG. 6 is similar to that of FIG. 3, with the exceptionthat there is no Table 36 relating RMS load voltage Vl to predictedcurrent Ip', and there is no storage of a variable W. The process ofFIG. 7 is similar to that of FIG. 4A, with the exception that steps 44and 46 are replaced by the single step 45 of taking the measured loadcurrent Io from the I/O port 28, and step 48 is modified as shown instep 48' to compute the voltage drop across the dimmer and cableresistances Rd, Rc directly from the measured load current Io, so thatthe desired switched output RMS voltage Vs is determined from theequation:

    Vs=Vl+Vd+Io(Rd+Rc)

It will be appreciated that, in order to permit the system to compensatefor voltage drops and be able to supply the rated voltage, say 240V, tothe loads, the input supply voltage must be greater than the ratedvoltage. This is achieved by supplying power through an auto-transformerwhich steps up the supply voltage from, for example, nominally 240V to264V, or by using a special high voltage mains supply of, for example,264V.

The controlling operations of the dimmer system have been describedabove, but it will be appreciated that the system must firstly beinitialised to set up the common Tables 36, 38, the common variable f,the table 34 for each dimmer, and the variables Rc, Rd, Vd for eachdimmer, and the initial load value W for each dimmer. The tables 34 to38 may be stored in non-volatile memory associated with each dimmerprocessor 18. Alternatively, they may be stored in non-volatile memoryassociated with the common processor 12 and be down-loaded to the dimmerprocessors in an initialisation process. In this case, the dimmertransfer function Table 34 to be used for each dimmer may be selected,using the terminal 14, from any of a set of different tables providing,for example, a square-law transfer function, a linear function, aconstant function, or a specially programmed function The mainsfrequency value f may be measured by the dimmer processor 18 or by amains processor 88 (FIG. 1) connected across the mains supply L-N andsupplying the frequency value f to the I/O ports 28 of the dimmerprocessors either merely during the initialisation process, orrepetitively during the operation of the system. The values Rc, Rd, Vdand W for each channel may be entered by the terminal once the system iscommissioned and stored in non-volatile memory associated with thecommon processor 12, and then be down-loaded to the dimmer processors 18each time the system is initialised. Alternatively, these values may besent to the dimmer processors when the system is commissioned and storedin non-volatile memory associated with the dimmer processors.

In the system described above, it has been assumed that the Table 38relating desired switched output RMS voltage Vs to required firing angleA is an invariable table. In one modification, in order to compensatefor variations in the mains RMS voltage, the voltage Vs used as theaddress for Table 38 may be scaled by a factor of Vr/Vm, where Vr is therated mains RMS voltage and Vm is a measured value of the actual mainsRMS voltage. Whilst this may be satisfactory for some applications, itwill be appreciated that other perturbations in the mains supply willcause variations in the required firing angle A to produce a desiredswitched output RMS voltage Vs.

Referring to FIG. 8, a nominal mains half wave cycle is denoted byreference numeral 90 and is of perfect sine form, having a peak valuewhich is root-two times the rated RMS voltage. In practice, however,various errors arise in the mains wave form. Firstly, the voltage may begenerally low as shown by curve 92, or even high. Secondly, the peak ofthe wave may be suppressed due to saturation effects in the transformersof the supply network, as denoted by curve 94. Furthermore, in atheatre, or a television or film studio, where a large number ofdimmer-controlled loads are in use, a progressively larger load may beimposed on the mains as the mains half-cycle progresses, thus pullingdown the supply voltage as the half-cycle progresses, as shown by curve96. These various perturbations in the mains supply all effect theswitched output RMS voltage Vs which is, in fact, obtained for a givenfiring angle A. The mains processor 88 (FIG. 1) is included tocompensate for these perturbations by supplying to the dimmer processors18 data for the Tables 38 (FIG. 3) derived from measurement andprocessing of the mains wave form, rather than including in the Tables38 fixed theoretical data for a perfect form and amplitude of mainssupply wave.

Referring to FIG. 9, the mains processor 88 includes an input from themains L which is applied, through a low-pass analogue filter 98, whichremoves any high frequency interference on the signal, to an analogue todigital converter 100, which applies a digital voltage signal V to aninput/output port 102 for a processor 104. The processor 104 hasassociated ROM 106 and RAM including storage for three tables 108, 110,112 and for a variable f.

The process carried out by the processor 104 is illustrated in FIG. 10.In steps 114 to 118, a variable t is reset and the voltage value V isrepeatedly tested in a loop until a zero-crossing is detected in whichthe value V is substantially equal to zero. Then, the value of V isstored at an address corresponding to the time variable t in Table 108,in step 120. After a predetermined delay in step 122, the time variablefor t is incremented in step 124. Then, in step 126, a fresh value forthe voltage variable V is detected, and in step 128 it is tested whetherthe value V is substantially equal to zero indicating the end of ahalf-cycle period. If it is not, then the process loops back to step120, where the value of the variable V is stored in Table 108 at anaddress t corresponding to the incremented time variable. It willtherefore be appreciated that while the loop of steps 120 to 128 isrunning the Table 108 is built up of the instantaneous voltage of themains over one half-cycle period. At the end of the half-cycle period,in step 130, the mains frequency f is computed from the equationf=1/(2t) and is stored in the RAM. Then, in steps 132 to 138, a loopprocess is performed for each value of firing angle variable A from pito zero, with a step of -pi/1024. In this loop, in step 134, the RMSvoltage Vs over the half-cycle period is computed for the voltagesignals in Table 108 between the time A/(2πf) and the time at the end ofthe half-cycle period, that is 1/(2f). In step 136, the computed RMSvoltage signal Vs is stored in the Table 110 at an address correspondingto the firing angle A. It will therefore be appreciated that once theTable 110 has been completed, it stores the switched output RMS voltagewhich will be obtained for any of 1024 firing angles A over thehalf-cycle period. In step 140, the processor 104 performs an operationto invert the Table 110 and store it as Table 122, in which requiredfiring angle A can be looked up for any required switched output RMSvoltage Vs. In step 142, the variable f is sent to the I/O port 102 fortransmission to the dimmer processors 18, and in step 144, the look-uptable 112 is sent to the I/O port 102 for transmission to the dimmerprocessors and storage as Table 38 in each of the dimmer processors (seeFIG. 3). Thus, each of the dimmer processors 18 has stored a look-uptable of firing angle A against switched output RMS voltage Vs which hasbeen derived by measuring the mains wave form, rather than a theoreticallook-up table.

Since the transmission of the Table 112 will entail heavy data traffic,either one of two modifications may be made to the process shown in FIG.10. In one modification, after step 142, a low-pass digital filterprocess is applied to the data in Table 112 prior to transmission inorder to reduce the amount of data. Then, when the Tables 38 are set upin the dimmer processors 18, an interpolation operation can be carriedout to obtain values of firing angle A for voltages Vs intermediate thevalues which have been transmitted.

In the second modification, in step 148, a delta process is applied tothe data in Table 112, so that rather than transmitting the absolutefiring angle value A for each voltage Vs, the difference between thatfiring angle value A and the previous firing angle value A istransmitted. Therefore, less bits of data will be required to be sent.

Referring to FIG. 1, a single mains processor 88 has been shown for allof the dimmer processors. In a modification to this arrangement, inorder to avoid the heavy amount of data traffic from the mains processor88, the mains processing may be carried out by each dimmer processor 18so that the Table 112 produced in the mains processing also serves asthe Table 38 for the dimmer processing.

It will be appreciated that in the case where a theatre or studio issupplied with a three-phase mains supply, then there will be differencesbetween the mains wave form on each of the three phases. In order toaccount for this difference, three mains processes may be carried out,one for each phase, and the dimmer processors may refer to theappropriate look-up table in dependence upon which phase is being usedto power the lighting load in question.

Whilst the embodiment of the invention described above utilises powercontrol by thyristors which are gated on and remain on for the remainderof the half-cycle, it will be appreciated that the invention is alsoapplicable in the case where gate turn-off thyristors are used, or inthe case where pulse-width-modulated switching devices are employed. Theinvention may also be put into practice using a variable resistor ortransformer for varying the power supplied to the load.

Reference is directed to United States patent application Ser. No.07/449,585, filed in the Patent and Trademark Office by Express Maildeposited Dec. 12, 1989 (now abandoned) the matter of which isincorporated herein by reference.

We claim:
 1. A lighting circuit, for connection to a power supply,comprising a lighting load, a power controller and means for connectingthe lighting load to the power supply via the power controller, thepower controller being arranged to apply an output voltage across theload via the connecting means to cause an output current to flow to saidload, there being a voltage drop along said connecting means, the powercontroller being operable to determine said output current and to supplyas said output voltage an RMS voltage which is greater than a desiredRMS voltage across the lighting load by an amount dependent upon saiddetermined current to compensate for said voltage drop along theconnecting means.
 2. A circuit as claimed in claim 1, wherein the powercontroller is operable to measure the output current and to vary theoutput voltage directly in dependence upon the output current thusmeasured.
 3. A circuit as claimed in claim 1, wherein the powercontroller comprises means for storing estimated size of the load data,and means for determining a predicted current signal from a desiredvoltage across the load and said estimate size of the load data, and isoperable to vary the output voltage in dependence upon said predictedcurrent signal, and is operable to measure the output current to theload and to update the estimated size of the load data in dependenceupon said output current and said predicted current signal.
 4. Acontroller for an electric lighting load, comprising:a controllableswitch for connecting an AC power supply having a supply cycle period toa load; and means for controlling the switch to conduct during ahalf-cycle of the AC supply for a conduction period less than thehalf-cycle period; characterized by: means for producing a currentsignal indicative of a current flowing to the load; and the switchcontrolling means being operable to increase the conduction period asthe current represented by the current signal increases.
 5. A controlleras claimed in claim 4, wherein the switch controlling means is operableto receive a desired level signal indicative of a desired output level,and including means to produce, from the desired level signal and thedetected current signal, a modified output level signal indicative of anoutput level greater than the desired output lever indicated by saiddesired level signal by an amount dependent on the detected currentsignal, and is operable to control the switch in accordance with themodified output level signal, to produce a switch output RMS voltagewhich varies substantially linearly with the load current.
 6. Acontroller according to claim 5, wherein said modified output levelsignal producing means is also operable to cause the modified outputlevel signal to represent an output level greater than the desiredoutput level by a predetermined amount independent of current.
 7. Acontroller as claimed in claim 4, in combination with a lighting load toform a lighting circuit.
 8. A controller as claimed in claim 5, incombination with a lighting load to form a lighting circuit having aresistance, and further comprising means to set the slope with which theswitched output RMS voltage varies with current in accordance with theresistance.
 9. A controller as claimed in claim 8, wherein the means forproducing the modified output level signal is also operable to cause themodified output level signal to represent an output level greater thanthe desired output level by a predetermined amount independent ofcurrent and further comprising means for setting said predeterminedamount in accordance with the voltage drop across the controllableswitch at substantially zero current.
 10. A controller as claimed inclaim 4, wherein the switch controlling means is operable to receive adesired output level signal indicative of a desired output level, andincluding means to store a stored load value indicative of the size of aload, means responsive to the desired output level signal and the storedload value to produce an expected current signal indicative of anexpected current to the load, means to produce from the desired outputlevel greater than the desired output level by an amount dependent onthe expected current represented by said expected current level signal,the switch being controlled in accordance with the modified output levelsignal to produce a switched output RMS voltage which variessubstantially linearly with expected load current, and means foradjusting the load value in accordance with the difference between theexpected and detected current signals.
 11. A controller as claimed inclaim 10, wherein the adjusting means has a time response period whichis greater than the cycle period of the AC supply.
 12. A method ofcontrolling power supplied from an AC supply to a lighting load by acircuit including a controllable switch, the method comprising the stepof:controlling the switch to conduct during a half-wave cycle of the ACsupply for a conduction period less than the half-cycle period toproduce an RMS output voltage; characterized by the steps of:determining the current flow to the load; and compensating in the switchcontrolling step to increase said RMS output voltage to compensate avoltage drop in the circuit due to the resistance of the circuit inaccordance with the determined current.
 13. A method as claimed in claim12, further comprising the step of compensating in the switchcontrolling step for a voltage drop across the controllable switch atsubstantially zero current.
 14. A lighting circuit, for connection to apower supply, comprising a lighting load, a power controller and meansfor connecting the lighting load to the power supply via the powercontroller, the power controller being arranged to apply an outputvoltage across the load via the connecting means to cause an outputcurrent to flow to said load, there being a voltage drop along saidconnecting means, the power controller being operable to determine saidoutput current and to supply as said output voltage an RMS voltage whichis greater than a desired RMS voltage across the lighting load by anamount dependent upon said determined current to compensate for saidvoltage drop along the connecting means, said power controller beingoperable to vary the output voltage in dependence upon a predictedcurrent to the load determined from the desired voltage across the loadand an estimated size of the load, and being operable to measure theoutput current to the load and to update the estimated size of the loadin dependence upon the measured and predicted currents.
 15. A controllerfor an electric lighting load, comprising: a controllable switch forconnecting an AC power supply to a load; andmeans for controlling theswitch to conduct during a half-cycle of the AC supply for a conductionperiod less than the half-cycle period; characterized by: means forproducing a signal indicative of a current flowing to the load; and theswitch controlling means being operable to increase the conductionperiod as the current represented by the current signal increases andbeing operable to receive a signal indicative of a desired output level;and further including means to store a value indicative of the size of aload, means responsive to the desired output level signal and the storedload value to produce a signal indicative of an expected current to theload, means to produce from the desired output level signal and theexpected current signal a modified output level signal indicative of anoutput level greater than the desired output level by an amountdepending on the expected current, the switch being controlled inaccordance with the modified output level signal to produce a switchedoutput RMS voltage which varies substantially linearly with expectedload current, and means for adjusting the load value in accordance withthe difference between the expected and detected current signals.
 16. Acontroller as claimed in claim 15, wherein the time response of theadjusting means is greater than the period of the AC supply.
 17. Acontroller as claimed in claim 15, wherein the means for producing theexpected current signal comprises a look-up table relating current tooutput level for a particular lighting load of a standard type, thecurrent looked-up from the table being scaled in accordance with thestored load value.
 18. A controller as claimed in claim 15, wherein themeans for producing the expected current signal comprises a look-uptable relating current to output level for a particular lighting load ofa standard type, the current looked-up from the look up table beingscaled in accordance with the stored load value.
 19. A controller for anelectric lighting load, comprising; a controllable switch for connectingan AC power supply to a load; andmeans for controlling the switch toconduct during a half-cycle of the AC supply for a conduction periodless than the half-cycle period; characterized by: means for producing asignal indicative of a current flowing to the load; the switchcontrolling means being operable to increase the conduction period asthe current represented by the current signal increases; and means forsetting the slope with which the switched output RMS voltage varies withthe current.
 20. A controller for an electric lighting load to beoperated at a predetermined voltage, comprising:voltage generating meansfor generating a voltage having a controllable RMS value, for supply tosaid electric lighting load, control means for controlling the voltagegenerating means to vary said RMS value of said voltage, and store meansfor storing voltage drop data indicative of a voltage drop to saidelectric lighting load, said control means being operable to read saidstore means and to control said voltage generating means to generatesaid RMS voltage to exceed said predetermined voltage by a voltage dropincrement set by said voltage drop data.